The present invention relates to a redox flow secondary battery useful for electricity storage and the like, more specifically to a redox flow secondary battery which is capable of long-term continuous operation and stable electricity storage.
Nowadays, atmospheric carbon dioxide concentration is substantially increasing due to massive consumption of fossil fuels and, consequently, global warming has become a serious problem. This problem has provided an impetus to the development of solar batteries as sources of clean energy. However, since solar batteries cannot generate electricity at night and in wet weather, high performance secondary batteries to be used in combination with solar batteries have been strongly needed.
On the other hand, conventional power plants are required to have generating capacities which meet the highest demands, suffering from a big difference in electricity demands between night and day, which decreases the operating rate of the power plants. Therefore, it is necessary to level out the operation load by storing electric power at night in large-sized electricity storage batteries and supplying the electric power in the day time to increase the operating rate of the power plants to enable effective operation. For this purpose, the development of large-sized electricity storage batteries is required. In addition, the development of a secondary battery with a higher energy density suitable as a power source for movable bodies such as electric vehicles is also required.
Redox flow secondary batteries are a promising candidate for a new secondary battery suitable for the above-mentioned applications because they can be charged flexibly matching with the output voltage of a solar battery by tapping (a method of using only necessary units out of many cell units depending on the demand) and have a relatively simple structure permitting upsizing.
A redox flow secondary battery uses liquid battery active materials and the positive and negative active materials are circulated in liquid-permeable electrolytic cells, where an oxidation-reduction reaction occurs which enables charge and discharge. Redox flow secondary batteries have the following advantages over conventional secondary batteries.
(1) It is possible to increase the storage capacity simply by increasing the volume of the storage tanks and the amount of the active materials and the electrolytic cell itself can be used without modification unless the output is increased.
(2) Redox flow secondary batteries have the positive and negative active materials stored completely separately in different containers, so there is a lesser possibility of self discharge than the batteries of other types in which two active materials are in contact with their corresponding electrodes.
(3) The charge and discharge reaction (electrode reaction) of active material ions entails only exchanging electrons on the surface of the liquid-permeable carbon porous electrodes used in the redox flow secondary batteries. Therefore, the active material components are not deposited on the electrodes unlike zinc ions in zinc-bromine batteries, meaning the cell reaction is simple.
Iron-chromium batteries, which are one of the redox flow secondary batteries that have been developed to date, have not yet become commercially practical due to the shortcomings of low energy density, and iron ion and chromium ion mixing through the ion exchange membrane.
Therefore, so-called xe2x80x98all vanadium redox flow secondary batteriesxe2x80x99, which have both the positive and negative electrolytes comprised of vanadium, have been proposed (J. Electrochem. Soc., 133, 1057 (1986), Japanese Patent Application Publication Laid Open No.62-186473). These batteries have several advantages in that they have higher electromotive force and energy density than iron-chromium batteries; they can be easily regenerated by charging even if the positive and negative electrolytes are completely mixed through the membrane because the electrolytes are based on a single element; and the electrolytes can be completely closed without decreasing the battery capacity.
However, during the charge/discharge cycling of the all-vanadium redox flow batteries, as with other kinds of redox flow batteries, there is a preferential volumetric transfer, which is due to the transfer behavior of various ions and water as a solvent in the electrolytes migrating through the membrane, in either of the positive/or negative electrolyte. This leads to a significant decrease in the battery capacity.
In order to solve the problem, extremely tiresome procedures are required in which the electrolytes in the tanks for the positive and negative electrolytes are mixed to adjust the compositions and amounts of the electrolytes to the initial state for every certain cycle of charge and discharge, before charge and discharge are started again. This mixing procedure is extremely tiresome itself and has a shortcoming in that it needs additional electric power in order to prepare the mixed electrolyte, which is a mixture of tetravalent vanadium and trivalent vanadium, for providing tetravalent vanadium for the positive electrode and trivalent vanadium for the negative electrode before charge and discharge are started again, sacrificing a large quantity of electricity. For commercialization, the interval between mixing the electrolytes has to be lengthened and the frequency decreased as low as possible in order to enable long-term continuous operation.
In view of this situation, the inventors have intensively investigated how to prevent the decrease in the electric capacity due to the migration of the liquid accompanied by charge and discharge in vanadium redox flow type secondary batteries and how to decrease the frequency of the conventional mixing procedure as low as possible to enable long-term continuous operation and, as a result, completed the present invention.
Accordingly, the present invention provides a redox flow type secondary battery which is a type of a liquid-circulating battery comprising a battery cell and storage tanks for positive and negative electrolytes, wherein the battery cell is separated by a membrane to provide a positive cell and a negative cell, each cell having a liquid-permeable porous electrode disposed therein, and the positive and negative electrolytes are passed and circulated from the storage tanks for the positive and negative electrolytes to the positive and negative cells, respectively, to conduct an oxidation-reduction reaction to charge and discharge the battery, characterized in that the positive and negative electrolytes are sulfuric acid aqueous solutions with vanadium ion concentrations of 0.5 mol/l to 8 mol/l and the electrolyte which migrates through the membrane over cycles of charge and discharge is returned from the storage tank where the liquid increases to the storage tank where the liquid decreases through a pipe in order to keep the change in the amounts of the positive and negative electrolytes in a certain range while charge and discharge are carried out.
Furthermore, the present invention provides a redox flow type secondary battery, wherein the amount of the liquid in the storage tank where the liquid decreases over cycles of charge and discharge is set greater than that of the liquid in the other storage tank beforehand prior to the operation.
Furthermore, the present invention provides a redox flow type secondary battery, wherein the relationship between the amounts of the liquids in the storage tanks for the positive and negative electrolytes and the type of the membrane satisfies one of the conditions (a) to (c):
(a) The membrane is an anion exchange membrane and the amount of the liquid in the storage tank for the positive electrolyte is set greater than that of the liquid in the storage tank for the negative electrolyte;
(b) The membrane is a cation exchange membrane and the amount of the liquid in the storage tank for the negative electrolyte is set greater than that of the liquid in the storage tank for the positive electrolyte; or
(c) The membrane is a multi-layered membrane obtained by laminating plural anion and cation exchange membrane layers and the amount of the liquid in the storage tank for either of the positive or negative electrolyte where the liquid decreases depending on the properties of the multi-layered membrane is set greater than that of the liquid in the other storage tank.
Furthermore, the present invention provides a redox flow type secondary battery, wherein the liquid level in the storage tank where the liquid increases over cycles of charge and discharge is set higher beforehand than that in the other tank and a liquid-refluxing pass is equipped for refluxing the liquid from the higher storage tank to the other tank with the aid of the gravity difference between the liquid levels.
Furthermore, the present invention provides a redox flow type secondary battery, wherein the relationship between the liquid levels in the storage tanks for the positive and negative electrolytes and the type of the membrane satisfies one of the conditions (1) to (3):
(1) The membrane is an anion exchange membrane and the liquid level in the storage tank for the negative electrolyte is set higher than that in the storage tank for the positive electrolyte;
(2) The membrane is a cation exchange membrane and the liquid level in the storage tank for the positive electrolyte is set higher than that in the storage tank for the negative electrolyte; or
(3) The membrane is a multi-layered membrane obtained by laminating plural anion and cation exchange membrane layers and the liquid level in the storage tank for either of the positive or negative electrolyte where the liquid increases depending on the properties of the multi-layer membrane is set higher than that in the other storage tank.
Furthermore, the present invention provides an overflow type battery in which there is a difference between the liquid levels in the storage tanks for the positive and negative electrolytes and the liquid is refluxed with the aid of the gravity difference between the liquid levels, characterized in that the membrane is a multi-layered membrane obtained by laminating plural anion and cation exchange membrane layers, and the cation exchange membrane layer faces the positive cell, and the liquid level in the storage tank for the positive electrolyte is set higher than that in the storage tank for the negative electrolyte.
Furthermore, the present invention provides a battery wherein the positive and negative electrolytes are sulfuric acid aqueous solutions of vanadium, and the concentrations of sulfate ions in the electrolytes are 0.5 mol/l to 9.0 mol/l.